In fetal lambs, substantial cardiovascular changes occur at birth. The birth process is accompanied by loss of the placental circulation, inflation of the lung, a rearrangement of the vascular system, a large increase in fetal oxygen consumption, and profound cardiac alterations, including revesal of right and left ventricular dominance, and a marked increase in ventricular output. We propose to systematically investigate the physiological mechanisms which are potentially responsible for the changes in ventricular performance at birth. We will study, longitudinally, unanesthetized near-term sheep fetuses in utero and following birth. From pre- and post-natal measurements of ventricular diastolic pressures, indices of contractility, and stroke volumes, the following will be determined: 1) ventricular "function curves" (relating stroke volume to ventricular end-diastolic pressure), 2) afterload sensitivities, 3) effects of coronary blood flow and levels of oxygenation, and 4) effects of heart rate and autonomic tone on ventricular function. We propose that increased myocardial oxygen supply and increased ventricular filling pressures, consequent to the onset of pulmonary respiration as well as cardiac sympathetic stimulation, are responsible for the marked increase in ventricular output at birth.